**12. Modification of the halogen bond donor**

The modification of the halogen bonds acceptor molecule for halogen bonding has been reviewed in many papers. As the above examples show a variety of acceptors can be used for halogen bonds. In general, it can be said that a good halogen bond acceptor is a strong electron donor. Much less have been written about the modification of the XB donor. [21,54]

Some of the strongest halogen bond donors are dihalogen molecules, which form strong halogen bonds.[55] This is of course due to the polarizing effect of the other halogen atom. All dihalogen molecules can act as halogen bond donors and the order of the XB bond strength follows the order I2> Br2> Cl2> F2. This is again the result of the polarizability of the halogens increasing in the same series.[20] The bond strength can be increased further by substituting the second halogen atom in dihalogens with fluorine, which polarizes the other halogen even more strongly. [56,57] The lighter dihalogens are volatile and so the crystal structures of such systems are rather rare. However, some of these structures have been characterized in gas phase by rotational spectroscopy.[58] The results indicate that for Cl2, ClBr, ClF and ICl the covalent halogen-halogen bond strength increase in the order Cl2 < BrCl < ClF < ICl. When combined with the known crystallographic data the increasing strength of the halogen bond donors for dihalogens can be given: F2< Cl2< Br2< I2< IBr < ICl. This order seems to be independent on the halogen bond acceptor.[54]

Polyhalides is another well known group of XB donors.[59] In these systems the halogen bonding typically occurs solely between the polyhalogenides and do not include other molecules. This is especially true with the polyiodides.[59] The polyiodide networks are often complicated three-dimensional networks, layers, or chains. The properties of these compounds have been intensively studied. There are even examples of systems where some of the iodines can be released into solution without breaking the crystal structure.[60] The removal of iodines have then been used to change the nonlinear optical properties of the compound.[9]

In the polyhalides iodine and bromine can often act both as halogen bond donors and as acceptors, and occasionally, the same atom can act both as acceptor and donor.[59] Amphoteric halogen bonds have also been found within polyhalide networks, though they are not very common.[8] In addition to the homonuclear polyhalides, mixed polyhalides are also known. The most common type is the mixed trihalide.[61,62]

Fig. 19 shows the typical features of the polyiodides. In this example, there are two crystallographically different I3 units, one of which has two nearly identical I-I bonds. The second I3 has unequal I-I bonds and it is closer to a I2-I motif. The two I3 units are then linked via I2 molecule. This is a typical example of a polyiodide structure. In this particular network the I2 molecule acts as a halogen bond donor and the I3 units act as acceptors.[12]

**Figure 19.** An example of a polyiodide network of (C28H20N4 Pt)2+ (I8)2-.[63]

158 Recent Advances in Crystallography

halogen bond acceptor.[54]

crystallographically different I3-

compound.[9]

second I3-

acceptors.[12]

[21,54]

**12. Modification of the halogen bond donor** 

The modification of the halogen bonds acceptor molecule for halogen bonding has been reviewed in many papers. As the above examples show a variety of acceptors can be used for halogen bonds. In general, it can be said that a good halogen bond acceptor is a strong electron donor. Much less have been written about the modification of the XB donor.

Some of the strongest halogen bond donors are dihalogen molecules, which form strong halogen bonds.[55] This is of course due to the polarizing effect of the other halogen atom. All dihalogen molecules can act as halogen bond donors and the order of the XB bond strength follows the order I2> Br2> Cl2> F2. This is again the result of the polarizability of the halogens increasing in the same series.[20] The bond strength can be increased further by substituting the second halogen atom in dihalogens with fluorine, which polarizes the other halogen even more strongly. [56,57] The lighter dihalogens are volatile and so the crystal structures of such systems are rather rare. However, some of these structures have been characterized in gas phase by rotational spectroscopy.[58] The results indicate that for Cl2, ClBr, ClF and ICl the covalent halogen-halogen bond strength increase in the order Cl2 < BrCl < ClF < ICl. When combined with the known crystallographic data the increasing strength of the halogen bond donors for dihalogens can be given: F2< Cl2< Br2< I2< IBr < ICl. This order seems to be independent on the

Polyhalides is another well known group of XB donors.[59] In these systems the halogen bonding typically occurs solely between the polyhalogenides and do not include other molecules. This is especially true with the polyiodides.[59] The polyiodide networks are often complicated three-dimensional networks, layers, or chains. The properties of these compounds have been intensively studied. There are even examples of systems where some of the iodines can be released into solution without breaking the crystal structure.[60] The removal of iodines have then been used to change the nonlinear optical properties of the

In the polyhalides iodine and bromine can often act both as halogen bond donors and as acceptors, and occasionally, the same atom can act both as acceptor and donor.[59] Amphoteric halogen bonds have also been found within polyhalide networks, though they are not very common.[8] In addition to the homonuclear polyhalides, mixed polyhalides are

Fig. 19 shows the typical features of the polyiodides. In this example, there are two

linked via I2 molecule. This is a typical example of a polyiodide structure. In this

units, one of which has two nearly identical I-I bonds. The

motif. The two I3-

units are then

units act as

also known. The most common type is the mixed trihalide.[61,62]

has unequal I-I bonds and it is closer to a I2-I-

particular network the I2 molecule acts as a halogen bond donor and the I3-

**Figure 20.** The halogen bonding in (C8H4Br2S6)2 IBr2.[64]

Fig. 20 provides an example of a structure containing a mixed trihalide. The structure contains halogen bonds between the trihalide and two dibromo tetrathiofulvalene molecules. Additionally, there are also bifurcated halogen bonds between the bromine and sulfur atoms of the tetrathiofulvalenes. The very similar halogen bond distances between the trihalide (Br-I-Br) and Br-atoms of tetrathiofulvalene indicate that the bond strengths are nearly identical for both of the BrBr contacts. In this case the Br-atoms dibromo tetrathiofulvalene act as the halogen bond donors and the trihalide as the acceptor.[64]

Organic compounds containing a C-X bond are often relatively easy to modify, which makes them attractive halogen bond donors. As discussed earlier, among the most commonly used XB donors are fluorinated iodobenzenes.[22,47,65] There is obviously a large number of halogen containing organic compounds that could be used as halogen bonding donors. For the purpose of crystal engineering, the interesting parameters of these compounds are the geometry and expected strength of the halogen bonds. If the halogen atom is only singly bonded to a carbon atom, the formed sigma hole will be pointing in the opposite direction of that bond.[22] Hence, the direction of the halogen bond is clear and easy to predict. However, controlling the strength of the halogen bond donor requires further information. Numerous studies of this topic have been published, especially on aromatic halogen bond donors.[24,55–57,66] On the basis of both the existing experimental and theoretical studies, it can be stated that electron withdrawing substituents increase the strength of the halogen bond donor, while the electron-donating groups reduce it.[24,55,66]

**Figure 21.** Halogen bonding of the iodine atoms for (C12H13F3 I+)(CF3O3S- ) (a); (C11H12Cl2I+)(CF3O3 S- ) (b) and 2(C13H12F6I+) 2(CF3O3S - ) CH2Cl2 (c) [67]

In Fig. 21 there are three structures each of which contains the same basic building blocks of the alkenyl(aryl)iodonium trifluoromethanesulfonate salts, while one of them also contains dichloromethane. From the point of view of halogen bonds, these provide a useful illustration of the effects that the electron withdrawing groups have on the halogen bonding donor. In the first compound (a) there is one –CF3 substituent on the aromatic ring. The halogen bonds are formed between the iodine of alkenyl(aryl)iodonium cation and oxygen atoms of the triflate. The I-O distances are 2.910 Å and 2.991Å. In the second structure (b) there are two chlorine substituents on the aromatic ring, which makes it more electron-deficient than the previous one. There are now two crystallographically independent dimers (only one is shown in Fig. 21.) that are involved in similar type of halogen bonding between the iodines and triflates. However, the iodine-triflate distances are different. The IO distances are 2.848 Å and 2.802 Å for the first dimer and 2.832 Å and 2.850 Å for the second. The halogen bond distances for the structure (a) are clearly shorter compared to the distances of the (b) structure as one might expect based on the electron withdrawing substituents. The third compound (c) in Fig. 21 contains two –CF3 substituents on the aromatic ring, making it the most electron deficient of the three. Again there are two crystallographically independent dimers (only one is shown if Fig. 21) that have slightly different geometries. Despite of this the basic halogen bonding geometry involving the iodine atoms is similar with slightly different IO distances (2.767Å and 2.985Å for the first dimer and 2.881Å and 2.893Å for the second). Now the message obtained from the halogen bond distances is not so obvious. Although the shortest distance in (c) is clearly the shortest of them all, the variation of the distances is large. This is a useful reminder that the final solid state structure is a result of several competing interactions and conclusions based only on distances is often an oversimplification and may be misleading.[67]

160 Recent Advances in Crystallography

XB donors are fluorinated iodobenzenes.[22,47,65] There is obviously a large number of halogen containing organic compounds that could be used as halogen bonding donors. For the purpose of crystal engineering, the interesting parameters of these compounds are the geometry and expected strength of the halogen bonds. If the halogen atom is only singly bonded to a carbon atom, the formed sigma hole will be pointing in the opposite direction of that bond.[22] Hence, the direction of the halogen bond is clear and easy to predict. However, controlling the strength of the halogen bond donor requires further information. Numerous studies of this topic have been published, especially on aromatic halogen bond donors.[24,55–57,66] On the basis of both the existing experimental and theoretical studies, it can be stated that electron withdrawing substituents increase the strength of the halogen

bond donor, while the electron-donating groups reduce it.[24,55,66]

**Figure 21.** Halogen bonding of the iodine atoms for (C12H13F3 I+)(CF3O3S-

) CH2Cl2 (c) [67]

In Fig. 21 there are three structures each of which contains the same basic building blocks of the alkenyl(aryl)iodonium trifluoromethanesulfonate salts, while one of them also contains dichloromethane. From the point of view of halogen bonds, these provide a useful illustration of the effects that the electron withdrawing groups have on the halogen bonding donor. In the first compound (a) there is one –CF3 substituent on the aromatic ring. The halogen bonds are formed between the iodine of alkenyl(aryl)iodonium cation and oxygen atoms of the triflate. The I-O distances are 2.910 Å and 2.991Å. In the second structure (b) there are two chlorine substituents on the aromatic ring, which makes it more electron-deficient than the previous one.

(b) and 2(C13H12F6I+) 2(CF3O3S -

) (a); (C11H12Cl2I+)(CF3O3 S-

)

In most cases metal bound halogens act as halogen bond acceptors.[44,68] There are some examples where the interaction seems to be more amphoteric, but these are relatively rare cases.[69–71] In general, using metal centers to form synthons for halogen bonding networks can be beneficial, because they readily permit the formation of well defined geometries. In addition, by changing the oxidation state of the metal, the geometry and chemical properties of the system can also be changed.[72] Metal compounds can also possess interesting magnetic and luminescent properties.[46,73]

**Figure 22.** The halogen bonding network of PtCl2(C5NBrH4)2.[70]

In Fig. 22 there is an example of a network consisting of PtCl2(NC5H4Br)2 linked together via halogen bonds. In addition to the halogen bonds, the two dimensional layers consist of weak hydrogen bonds and π-π stacking interactions. The structure shown is a good example of a network structure of a metal complex formed by halogen bonding.[70]
